Polyamide fibers represented by polycaproamide and polyhexamethylene adipamide, and polyester fibers represented by polyethylene terephthalate and polybutylene terephthalate are being widely used, not only for clothing uses, but also for interiors, vehicle interiors, industrial uses, etc. because of excellent mechanical properties and dimensional stability thereof.
In the melt spinning of an ultrathin fiber for the purpose of giving softness to fiber, a fiber whose single-yarn diameter is of micron size can be obtained by melt spinning a single polymer. However, with regard to an ultrathin fiber that cannot be obtained by melt spinning a single polymer, it is mainstream practice to obtain a composite section fiber by the composite spinning of polymers of different solubilities and obtain an ultrathin fiber by performing a fiber-splitting treatment or dissolving and removing the polymer whose solubility is high.
For example, Japanese Patent Application Publication Nos. 2003-336128 and 2006-322131 show as examples a split type composite fiber comprising a polyamide component and a polyester component. The main focus of that fiber is to carry out a chemical fiber splitting treatment with a swelling agent to achieve the uniform division between the polyester component and the polyamide component. Besides, this split type composite fiber is intended to split the polyester component and the polyamide component, and dissolution of the polyester component is not the main focus. Therefore, in the case where the polyester component is dissolved and removed to obtain an ultrathin polyamide fiber, an alkali treatment requires a considerable amount of time, becoming a factor for increased cost. To reduce the alkali treatment time, it is possible to use an alkali solution at high concentration. In reality, however, the alkali solution treatment requires that measures against pollution of working environment and environment pollution be sufficiently taken due to issues in safety of agents. Besides, the working cost of the alkali solution treatment is high, and an increase in concentration of the alkali solution cannot be said to be a favorable method for industrial production in terms of working safety, environmental safety, and cost increases. Besides, the polyester component, being poor in alkaline elutability, has problems of not dissolving entirely, but being likely to partially remain undissolved.
Japanese Patent Application Publication No. 2005-194681 shows, as an example of an easily alkaline-elutable polyester component in a split type composite fiber comprising a polyamide component and a polyester component, a polyester obtained by copolymerization of an aromatic dicarboxylic acid component having a sulfonic acid salt group in a dicarboxylic acid component and polyalkylene glycol having an average molecular weight of 1000 to 10000. However, such a fiber, while having an alkaline elutability, is poor in compatibility with polyamide so that at the time of the yarn-making of a core-sheath composite fiber, separation of inner interface of composite fiber is likely, giving rise to a problem of scatter of fluff.
It could therefore be helpful to provide a core-sheath composite fiber that is able to restrain the separation of inner interface of composite fiber and therefore considerably better fluffing and yarn breakage at the time of the yarn-making of a polyester-and-polyamide core-sheath composite fiber, and that considerably improves the characteristic of alkaline elution of polyester, and a woven or knitted fabric, and also an ultrathin polyamide fiber obtained by dissolving and removing polyester contained in the core-sheath composite fiber, a woven or knitted fabric, and a fibrous product.